2 files changed, 11 insertions, 3 deletions
diff --git a/src/libstore/outputs-spec.cc b/src/libstore/outputs-spec.cc
index a9e4320d5..096443cb2 100644
--- a/src/libstore/outputs-spec.cc
+++ b/src/libstore/outputs-spec.cc
@@ -21,7 +21,8 @@ bool OutputsSpec::contains(const std::string & outputName) const
 
 std::optional<OutputsSpec> OutputsSpec::parseOpt(std::string_view s)
 {
-    static std::regex regex(R"((\*)|([a-z]+(,[a-z]+)*))");
+    // See checkName() for valid output name characters.
+    static std::regex regex(R"((\*)|([a-zA-Z\+\-\._\?=]+(,[a-zA-Z\+\-\._\?=]+)*))");
 
     std::smatch match;
     std::string s2 { s }; // until some improves std::regex
@@ -42,7 +43,7 @@ OutputsSpec OutputsSpec::parse(std::string_view s)
 {
     std::optional spec = parseOpt(s);
     if (!spec)
-        throw Error("Invalid outputs specifier: '%s'", s);
+        throw Error("invalid outputs specifier '%s'", s);
     return *spec;
 }
 
@@ -65,7 +66,7 @@ std::pair<std::string_view, ExtendedOutputsSpec> ExtendedOutputsSpec::parse(std:
 {
     std::optional spec = parseOpt(s);
     if (!spec)
-        throw Error("Invalid extended outputs specifier: '%s'", s);
+        throw Error("invalid extended outputs specifier '%s'", s);
     return *spec;
 }
 
diff --git a/src/libstore/tests/outputs-spec.cc b/src/libstore/tests/outputs-spec.cc
index c9c2cafd0..836ba7e82 100644
--- a/src/libstore/tests/outputs-spec.cc
+++ b/src/libstore/tests/outputs-spec.cc
@@ -40,6 +40,13 @@ TEST(OutputsSpec, names_out) {
     ASSERT_EQ(expected.to_string(), str);
 }
 
+TEST(OutputsSpec, names_underscore) {
+    std::string_view str = "a_b";
+    OutputsSpec expected = OutputsSpec::Names { "a_b" };
+    ASSERT_EQ(OutputsSpec::parse(str), expected);
+    ASSERT_EQ(expected.to_string(), str);
+}
+
 TEST(OutputsSpec, names_out_bin) {
     OutputsSpec expected = OutputsSpec::Names { "out", "bin" };
     ASSERT_EQ(OutputsSpec::parse("out,bin"), expected);